Architects and engineers that design HVAC systems for commercial buildings and other structures go to great lengths to ensure that those systems provide a consistent and reliable level of comfort to the occupants of those structures. HVAC designers carefully size the HVAC units to ensure delivery of the appropriate volume of conditioned air. Additionally, they design the ductwork to distribute the conditioned air to the various rooms and other areas of the structure at adequate volumetric rates. Furthermore, the designers select the spacing and configuration of the diffuser or register terminals through which air flow is discharged (hereafter referred to generally as “terminals”) to distribute and disperse the conditioned air into the rooms/areas in a predetermined manner so as to provide the desired level of comfort for the occupants.
Integral to this design is the need for the conditioned air to be dispersed from each terminal at a volumetric flow rate that is at or within a predetermined range of a rate specified by the designer. Flow rates that deviate from those specified by the designers will result in room or area conditions or comfort levels that deviate from the target, which can unnecessarily increase energy costs.
When new commercial HVAC systems are commissioned, the system requires balancing to ensure that the conditioned air is delivered from each terminal at a volumetric flow rate that is at or within a range specified by the system designers. Balancing can also be required as a part of routine HVAC system maintenance or when the floor plan within a building is reconfigured.
Balancing a commercial HVAC system is not a trivial matter and requires the services of a qualified HVAC technician. Commercial HVAC duct runs can be complicated and can have many trunks, branches, or zones, each of which has many terminals, or nodes. Not only does each terminal have its own damper for adjusting flow through that particular node, there are also dampers within the ductwork that can be used to control air flow to the various trunks, branches, and zones within the system. Once one considers that adjusting the flow through any one terminal within the HVAC system will necessarily create a change in backpressure that affects the air flow through all other terminals in the system, the complexity of the balancing task becomes clear.
Ceiling mounted terminals of commercial HVAC systems are selected by the system designer from a finite number of configurations to diffuse and direct conditioned air into the building space in a predetermined pattern. While there are many different terminal configurations from which to choose, a vast majority of the terminal designs fall within or are based around a standard 24-inch by 24-inch footprint common to commercial drop ceiling tiles.
Energy efficiency is one of the most important criteria in the design process of an HVAC system. The goal of an HVAC system is to deliver conditioned air through each terminal in the system at a specified target flow rate that is sufficient to provide suitable comfort levels. Any structure in the system that restricts or otherwise inhibits air flow amounts to a loss that must be accounted for. Dampers and terminals are sources of significant losses in the in HVAC systems. Since terminals have a fixed configuration and structure, they introduce a constant loss, which cannot be overcome. Dampers, however, being adjustable to control flow at each terminal, introduce variable losses that, while significant, can be minimized through proper system balancing.
The losses introduced by dampers increase as the dampers are closed and thereby offer more flow restriction. Therefore, it is ideal not only to balance the HVAC system to achieve the specified flow rates at each terminal, but to do so while having the dampers as open as possible so that losses are at a minimum.
The National Environmental Balancing Board (“NEBB”) is an international certification association that, among other functions, certifies individuals and firms to commission, test, adjust, and balance HVAC systems. In addition to certifications, NEBB also provides equipment specifications and procedural standards. On the equipment side, one piece of equipment for which NEBB issues specifications is referred to a direct reading hood, which is used to measure air flow through a ceiling mounted terminal. In this description, the more generic term “air flow hood” is used to describe a most commonly used form of a direct reading hood device. Those skilled in the art will appreciate that “direct reading hood” and “air flow hood,” as used in this description, are essentially interchangeable, i.e., the air flow hood described herein can be characterized as a direct reading hood within the NEBB specification.
Air flow hoods are instruments that are used by HVAC technicians to measure the air flow discharged through ceiling mounted terminals of commercial HVAC systems. Air flow hoods are designed to be held in place over the terminal. The hood acts as a duct that collects and redirects the air that is discharged from the terminal. The air flow hood has the configuration of a converging-diverging nozzle with a throat through which the conditioned air is directed in order to measure its volumetric flow rate. Velocity pressure is measured via instrumentation, such as an averaging pitot tube manometer located in the throat, used to calculate flow in a known manner.
HVAC technicians use these measured flows to balance the system by a method referred to in the art as proportional balancing. The basic principle of proportional balancing is that once set, the quantity of airflow from each terminal in a system will always remain in the same ratio or proportion to the other terminals in the system. Although the total quantity of the system changes, terminals will stay in the same percentage-of-flow relationship to each other. Although based on science, traditional proportional balancing relies on the experience of the technician to estimate terminal adjustments that will result in proportional balance.
To proportionally balance a system, initial flows are measured at each terminal. Percent of design flow is calculated for each terminal as the measured flow divided by design flow. The terminal that has the lowest percentage of design flow becomes the key terminal, which is left full open. Terminals are typically balanced in ascending order of percentage of design flow. With experience, however, a technician can balance terminals out of order.
The idea is to set the second terminal so that the percentage of design flow for that terminal and the key terminal are in the correct proportions. Once their percentages are in the correct proportions, they remain in the correct proportions. Although the airflow through these proportionally balanced terminals can and will fluctuate as other terminals are balanced, their percentage proportions will remain the same. To accomplish this, the second terminal is initially set based upon the knowledge and experience of the technician. In other words, it is an educated guess on the part of the technician. Flows at the second terminal and key terminal are re-measured to determine whether their percentage of design flows are within a predetermined tolerance. Once they are within this tolerance, the technician moves to the next terminal.
The process is repeated for each terminal in the system. The technician uses his knowledge to estimate the adjustment to each terminal so that it will result in its percentage of design flow being equal to the key and the other previously balanced terminals. Due to the reliance on the technician's estimation skills, adjusting, re-measuring, and repeating are frequent and common. Additionally, as the technician performs these balancing tasks, he also uses his knowledge and experience to estimate adjustments so that adjusting the final terminal will bring the percentage of design flow for that terminal and all others to not only be equal, but also as close to 100% as possible. Once accomplished, the technician adjusts fan speed, if necessary, to achieve 100% design flow for the terminals.
From the above, those skilled in the art will appreciate that traditional proportional balancing methods are inexact, time consuming, and prone to errors requiring re-adjustments. The system, method, and apparatus of the present invention eliminates this guesswork by systematically and scientifically determining set points for each terminal using mass flow theory, so that each terminal is set to a position that will result in system balance once the last terminal in the system is set.